Thermoplastic polyesters which are produced from aromatic dicarboxylic acids and aliphatic or cycloaliphatic glycols, especially polyethylene terephthalate, are widely used for fibers and films. Despite the desirable properties of polyethylene terephthalate, it has not generally been used as a molding material under the existing technological circumstances. In the past, this has been so because the rate of crystallization of polyethylene terephthalate is slow compared with other moldable crystalline polymers, and sufficiently crystallized molded articles could not be formed under normal injection molding conditions utilizing this polymer. Molding of polyethylene terephthalate under conventional molding conditions, for example, from 70.degree. to 100.degree. C. mold temperature, would bring about non-uniform properties, including a non-uniform appearance of the molded articles. Therefore, polyethylene terephthalate should be molded into articles using a mold temperature greater than 130.degree. C., and a longer molding cycle. However, this would cause a great decrease in overall working efficiency.
Another reason that polyethylene terephthalate has not generally been used in the past as a molding material is that it is inferior in toughness and has poor resistance to moist heat.
In order to improve the moldability of polyethylene terephthalate, it has been proposed to add various kinds of promoters of crystallization, e.g. benzophenone, and nucleating agents, such as for example, inorganic powder, metal soaps, and a combination of them. However, even if these molding techniques are employed, polyethylene terephthalate cannot be sufficiently molded to any practical degree. Molding can be carried out at a comparatively low temperature but with the sacrifice of the molding cycle. Furthermore, even if the molding is carried out under such conditions, a damaged appearance of the molded articles would result.
In Japanese Patent Publication No. 48-4097, it is disclosed that lithium, sodium or barium salts of monocarboxylic acids are effective in shortening the cycle time in the molding of polyethylene terephthalate. This technique, however, is not effective in lowering the molding temperature.
In U.S. Pat. No. 3,368,995, it is disclosed that certain kinds of particular additives are recommended as nucleating agents of compositions comprising polyethylene terephthalate and glass fibers. The nucleating agents include carbon powders such as graphite and carbon black, oxides of the metals of Group II of the Periodic Table such as ZnO and MgO, sulfates such as CaSO.sub.4 and BaSO.sub.4, phosphates such as Ca.sub.3 (PO.sub.4).sub.2, silicates such as CaSiO.sub.3 and MgSiO.sub.3, oxalates such as calcium oxalates, stearates such as magnesium stearate, benzoates such as calcium benzoate, salicylates such as zinc salicylate, talc and the neutral clays.
On the other hand, various methods have been proposed in order to improve the toughness of polyethylene terephthalate as a molding material. The method shown in Japanese Patent Publication Kokai No. 52-32045 indicates that a composition of .alpha.-olefins and glycidyl esters of .alpha.,.beta.-ethylenically-unsaturated carboxylic acid copolymers blended with polyesters is sufficiently effective to impart toughness to polyesters. This composition, however, cannot be effectively employed for injection molding of materials because of its longer molding cycle as compared to the molding cycle of polyester alone. Therefore, the improvements in the moldability of compositions comprising polyethylene terephthalate and the above-mentioned olefinic copolymers are greatly needed.
Polyethylene terephthalate resins have still another weak point in that they are subject to hydrolysis reactions which result, in time, in a degradation of their properties in the presence of moisture: the long polymer chain is degradated to a progressively lower molecular weight, and its strength and flexibility are reduced. Therefore, an improvement in the resistance to hydrolysis (moist heat) of polyethylene terephthalate is strongly desired.
In order to improve the crystallization characteristics of polyethylene terephthalate, it is necessary to expand both the upper and lower temperature regions in which crystallization can be induced, and to improve the crystallization rate.
As one approach, it is desired that the Tc is raised and the Tcc is lowered (Tc and Tcc representing the crystallization peak temperatures under the lowering and raising processes, respectively, as indicated by the Differential Scanning Colorimeter). The polymer indicating a higher Tc would be molded with a shorter molding cycle. The lower Tcc would make it possible for a lower molding temperature to be employed. In the conventional methods for the molding of polyethylene terephthalate resins, neither of the above requirements have been satisfactorily attained.
As a result of a detailed investigation on the crystallization characteristics of compositions comprising polyethylene terephthalate and the copolymers of .alpha.-olefins and glycidyl esters of .alpha.,.beta.-ethylenically-unsaturated carboxylic acids, already known as modifiers which impart toughness to polyesters, it has been discovered that the above-mentioned copolymers extended the proper crystallization temperature region of polyethylene terephthalate downward and made the compositions easy to crystallize even at lower temperatures. However, this blend of the copolymers lowers the upper limit of the crystallization temperature region at the same time. Consequently, the blended composite of these two ingredients has a characteristic property of crystallizing at lower temperatures, but of being difficult to crystallize as a composition compared with polyethylene terephthalate alone.
It has now been found that the above-stated problems can be dramatically solved by adding barium salts of fatty acids to a composition including the above-mentioned two polymers. Barium salts of fatty acids lead to a more rapid crystallization rate of the composite material, contributing to increase the crystallization rate of the lower temperature region synergistically with the olefinic copolymer and further to extend the upper region of the crystallization temperature.
In addition, barium salts of fatty acids promote the reaction between the polyethylene terephthalate and the glycidyl esters of .alpha.-olefin, and .alpha.,.beta.-ethylenically-unsaturated carboxylic acid copolymers on the interface of both ingredients, resulting in a more effective action of the copolymer which causes as enhancement in the mechanical properties and resistance to moist heat of the molded articles.